thickness: Number

Sets the thickness of the stripLine in pixels. This property should be used along with “value” property – it gets ignored when startValue & endValue are used.

var  chart =  new  CanvasJS.Chart("container",
{
 .
 .
axisY:{
    stripLines: [{
        .
        thickness:3,
        .
    }]
},
 .
 .
});
chart.render();